Since my husband had also stated that he was "cutting her off", I asked if he would call her either in front of me, or with me on the other line. He agreed to call her and "end it" with me sitting next to him on the couch. While I couldn't hear every single thing she said, I heard her crying and sobbing and begging. I also heard him tell her he chose me, and that what they had was a mistake.
Sure, he could call her the next day and tell her that it was all just an act for my benefit. If that's something your husband would do, then this isn't worth it. But for me, it helped alot to hear some of the things he said to her, and honestly, though I know it's a bit evil, it felt good to hear her in some pain.